In order to read data from or write data onto a disk media surface of a disk drive, a read/write head must be positioned precisely over a track of the media surface from which data is read or on which data is written. Failure to accurately position the read/write head over the desired track during a read operation results in inaccurate data retrieval. If the read/write head is improperly positioned during a write operation to the disk, not only is the written data lost, but data on adjacent tracks may be written over and destroyed. The accurate placement of the read/write head is therefore crucial to the utility of disk systems.
There are two general methods of positioning the read/write head over a particular track in order to retrieve or store data. In the first head positioning method, no head position feedback is provided. Instead, a stepper motor responds to a distance signal to shift the head a specified number of tracks. The disk drive does not sense the present position of the head. The reliability of this method for reading and writing data depreciates greatly as the tracks become more closely packed.
The second head positioning method utilizes head position data written on the disk surface to provide position feedback to the disk head controller in order to precisely position the read/write head in the center of the data track. Systems utilizing this method of head position control generally fall into two categories, servo-surface-servo-systems and data-surface-servo-systems.
The primary difference between the two servo-system approaches is the number of disks containing reference or servo information. In servo-surface-servo-systems, not the subject of the present invention, disk reference or servo information is stored on one surface of the disk. However, in data-surface-servo-systems to which this invention applies, each disk surface contains reference or servo information.
In data-surface-servo-systems writing servo data onto each disk surface occurs during the manufacturing process. A single pre-amplifier is interfaced to several disk surfaces and reads or writes to only one track of one disk surface at a time. When the servo-writing system writes to only one track per disk rotation, a substantial amount of time is consumed while the read/write head waits for the disk to reach a proper position before transmitting data. This servo-writing process takes anywhere from ten minutes for a single surface to a few hours for a fully embedded multiple surface system. When the servo-writing takes a few hours, the servo-surface preparation stage becomes a serious bottleneck in the hard disk manufacturing process. Eliminating or decreasing the severity of this bottleneck requires either an increase in the number of servo-writing devices or a decrease in the time required to write the servo data onto the disk surfaces.
Prior art systems have increased the number of pre-amplifiers and active read/write channels to exploit parallelism and decrease the time necessary for servo-writing each set of disks contained in a disk unit. Of course, the addition of multiple channels to the servo-writing system results in a proportional increase in throughput. However, the cost and complexity of the system also increases in proportion to the number of active writing channels. Furthermore, the likelihood of malfunction increases as the complexity of the servo-writing system increases. The increased cost and probability of malfunction of the multiple channel servo-writing device balances against any time benefits realized by increased parallelism in the servo-writing system. Furthermore, since the disk drive itself is capable of writing onto only one surface at a time, the multiple channel servo-writing systems must use external writing heads. These heads are used only during the servo-writing step in the manufacturing process and not during the normal operation of the disk drive. In prior art servo-writing systems the use of external heads to write servo data onto a disk surface decreased the quality of the servo data later read by the disk drive's own head due to incomplete compatibility between the servo-write device and the disk data retrieval device.